Legislative changes coupled with various recycling laws mandated by many state and local governments to increase wastepaper consumption, and the economical benefits that can be realized by utilizing wastepaper, particularly in the newsprint market, have put a burden on papermakers to raise the wastepaper content of their furnishes from a mere 10% to as high as 100% in certain grades of paper; specifically in the deinked newsprint market segment, where, because of availability and low cost, waste newsprint is particularly considered an important feed-stock.
To address these environmental issues as well as to take advantage of economic incentive, many newsprint mills using virgin pulps and unprinted pulp substitutes have been modifying their paper furnish by utilizing deinked newsprint fiber from old newspaper (ONP) and old magazines (OM)in various proportions.
Many mills have even installed integrated deinking facilities on-site which allow them to remove ink from the fiber and other unwanted foreign contaminants by various deinking processes such as washing and/or flotation. The precursor fiber material utilized in a deinking fiber furnish for newsprint may consist of a blend of 60-80% old newspaper (ONP) and the remainder old magazines, and after deinking, this mixture of deinked fiber furnish may constitute as low as 10% of the paper machine furnish (and the remainder virgin groundwood fiber) and as high as 100% deinked stock with no virgin groundwood fiber constituent. A mill's paper machine furnish may even constitute deinked 100% ONP.
The fiber furnish (FF) referred to here is defined as a mixture of precursor fibrous materials consisting of old newspaper (ONP) and old magazines (OM) in a certain mix ratio (such as 70/30) commonly utilized in a deinking process whereas paper machine furnish (PMF) is defined as a mixture of fiber stock consisting of deinked fiber furnish and virgin groundwood fiber along with other desirable papermaking chemical additives utilized in the papermaking process.
Regardless of whether the recycled fibers used in the papermaking furnish are purchased from outside sources or processed on-site via a deinking process, the furnish is not devoid of undesirable contaminants which, if not properly controlled or removed, would severely interfere with the subsequent papermaking processes and ultimately cause production loss and lower quality. The contaminant types, their magnitude, and their adverse effects on the quality and subsequent papermaking process depend on many factors such as deinking efficiency, types of chemical additives used, operating conditions, precursor fiber materials and their mix ratios in fiber and paper machine furnishes, etc.
As these mix ratios of ONP/OM in fiber furnish (FF) for deinking and the deinked fiber vs. virgin groundwood fiber in paper machine furnish (PMF) change, the nature and magnitude of contaminant-related problems, particularly in the press section of a papermaking process, also change. For example, a paper machine furnish (PMF) based on 100% deinked stock may pose no significant pitch deposition problems but may create severe problems related to felt discoloration and excessive deposition of residual ink particles, coating contaminants and fines in the press felt Structure, while a paper machine furnish containing relatively high levels (60% or more, for example) of Southern pine-type mechanical pulp fiber and the remainder deinked fiber furnish may create multiple problems mainly related to pitch, stickles from the use of coated magazine wastepaper and significant ink particle depositions in the press felts as well as on the press section machine components such as the uhle boxes and rolls.
One of the most severe problems associated with the high content level of Southern pine groundwood fiber (laden with virgin pitch) in the paper machine furnish is uhle box deposition in the press section of the paper machine. If proper adjustments in chemical conditions are not made and felts are not adequately conditioned, these contaminants tend to deposit on the uhle box surface (due to extreme shear) and thus requires frequent clean-ups in order to prevent vacuum loss, felt degradation and eventual loss of water removal capability of the press felts.
For a felt to maintain its useful function on a paper machine, it must be dewatered and cleaned as it passes around the paper machine prior to re-entering the press nip. This is commonly accomplished by the use of uhle boxes. Also, for efficient press section operation, the efficiency of the uhle boxes is extremely important in the removal of contaminants from the felt on a continuous basis without damaging the felt surface fibers. If uhle box deposition is not controlled effectively, the aforementioned deposits in felts and on uhle box surfaces tend to damage the surface batt fibers of felt, resulting in streaking, premature removal of expensive felt(s) and ultimately production loss and lower quality. Uhle box deposition problem is often addressed by machine operators manually scraping off the deposits from the uhle box surface on a regular basis which can pose a significant safety hazard, as well as causing problems due to particles falling on the felts or paper sheet.
Another common runnability and quality problem for mills using high levels of coated magazine in the ONP/OM fiber furnish is the "stickies" problem which deposit in felts and paper machine components or show on the sheet as spots. The primary sources of stickies may be contact adhesives, tapes, labels, decals, hot melts, seam binding, wax, ink, latexes, wet strength resins, etc. which generally emanate from high level of OM content in the furnishes. The use of coated grade paper in the fiber furnish may also introduce sticky substances composed mainly of latex binder (coating mixture of PVAC, SBR, TiO.sub.2, CaCO.sub.3, other inorganics) which agglomerate into stickies. Stickies are prone to deposit on uhle boxes, press rolls, dryer cans, press felts, dryer fabrics, etc., which ultimately cause sheet holes/spots besides productivity loss. These problems are much more severe in mills that have closed paper machine white water systems.
The use of 100% deinked fiber as paper machine furnish is becoming quite popular with a different set of problems related to ink and their deposition potential in press felts.
Newsprint inks are composed of two main types: so called letter-press ink and offset ink. Both types are a complex mixture of ingredients including pigments which are responsible for providing color (tend to discolor the felt); vehicles which are responsible for transferring the pigment via press to the paper and holding them (bonds to the felt batt fibers), and the modifiers to achieve specific end-use physical properties. Majority of the printing inks contain carbon black as pigment, with and without organic pigment (also called toner) more commonly employed in color printing of magazines. Vehicles (also called binders and are organic component often with ionic group) part of the ink may contain one or more vegetable drying oil, mineral oil, varnish and solvents, lacquers, shellacs, acrylic and other polymeric emulsions, nitro and other cellulosic derivatives. Modifiers could contain components such as clay, waxes, rosin, glycol, gums, rubber defoamers, silicones, etc.
Typical deinking chemical additives employed during a deinking process may include sulfuric acid, sodium hydroxide, sodium silicate, hydrogen peroxide, surfactants, chelants (such as DTPA-diethylene triamine penta acetic acid), and calcium chloride, etc. each with specific function to perform at various stages of the deinking process. The carryover residuals of these well intended chemicals in fiber furnish sometimes aggravate other contaminants present in the subsequent papermaking process and show-up as problematic contaminants particularly in the press section of the papermaking system.
Another problem seriously affecting the press section felts is the use of recycled water as shower water where insoluble ultra-fine particles (fines) may cause serious felt filling problems if these fines are not removed by the uhle boxes. A high percentage of fabrics and felts taken off in mills using deinked fibers are removed because they have been partially filled with fines and other contaminants. The insoluble contaminants are trapped by the tight fiber mat, filling the void volume of felts, and thus reducing their water removal capability.
All these undesirable fines and non-fibrous contaminants introduced in the paper machine system either by necessity or inherent with the fiber and/or paper machine furnishes are extremely difficult to eliminate completely and, therefore, they tend to deposit on the uhle box surface, and in the felt structure, thus reducing the void volume of the felt, excessive discoloration of felt in the case of high deinked fiber content in paper machine furnish, etc. and thus the ultimate result is poor runnability and quality problems.
Papermaking machines are well known in the art. The modern papermaking machine is in essence a device for removing water from the paper furnish. The water is removed sequentially in three stages or sections of the machine. In the first or forming section, the paper furnish is deposited on a moving forming wire and water drained through the wire to leave a paper sheet or web having solids content of 18-25% by weight. The formed fiber web is carried into a press section and passed through one or more roll nip presses on moving press felts to remove sufficient water to form a sheet. This sheet is transferred to the third stage known as dryer section of the paper machine. The present invention deals with the continuous conditioning treatments of press felts employed in the second stage known as the press section where the above-mentioned dispersed substances and/or small particulate impurities emanating from the use of deinked paper furnish (along with chemical additives ), if not effectively treated or retained in the sheet, would deposit in press felts, on uhle box, machine rolls and thus render felts and uhle box ineffective by reducing their water handling capabilities.
Because of a variety of multi-component contaminants (pitch, stickies, ink, deinking chemicals, etc. ) present in the deinked newsprint type paper machine furnish and various distinct problems arising from them (such as discoloration of felt, uhle box deposition, felt filling, etc.); various conventional reagents, solvents, surfactants, dispersants, wetting agents, etc. and their combinations employed in the prior art have shown very limited effectiveness toward addressing these multiple problems occurring in the press section of the papermaking process. In fact, many references cited in the prior art search deal only with the various aspects of deinking process chemistry and equipment involved but none with the chemical conditioning of felts and contaminants in a papermaking system utilizing deinked fiber. The present inventors have discovered that significant superior results with respect to felt cleanliness, inhibition of contaminants deposition in the felt structure as well as uhle box deposition can be obtained by applying the felt conditioning treatments according to the teachings of this invention.